The manufacture of very small lenses has undergone enormous development since the 17th century when Robert Hooke and Antonie van Leeuwenhoek both developed techniques to make small glass lenses for use with their microscopes. In these early techniques, Hooke painstakingly melted small filaments of glass and allowed the surface tension in the molten glass to form the smooth spherical surfaces required for lenses. (See, Hooke R, Preface to Micrographia, The Royal. Society of London (1665), the disclosure of which is incorporated herein by reference.)
Advances in technology have enabled microlenses to be designed and fabricated to close tolerances by a variety of methods. The optical efficiency of diffracting lenses depends on the shape of the groove structure and, if the ideal shape can be approximated by a series of steps or multilevels, the structures can even be fabricated using technology developed for the integrated circuit industry. This area is known as binary optics. (See, e.g., Veldkamp W B, McHugh T J., Scientific American, Vol. 266 No. 5 pp 50-55, (May 1992), the disclosure of which is incorporated herein by reference.) In most cases, multiple copies of these lenses are desired for use in large lens arrays. These lens arrays can be formed by moulding or embossing from a master lens array. The ability to fabricate arrays containing thousands or millions of precisely spaced lenses has led to an increased number of applications. (See, e.g., Borrelli, N F. Microoptics technology: fabrication and applications of lens arrays and devices, Marcel Dekker, New York (1999).
Indeed, microlenses in recent imaging chips have attained smaller and smaller sizes. The Canon EOS-1Ds Mark III packs 21.1 million microlenses onto its CMOS imaging chip, one per photosite, each just 6.4 micrometer across. An announced Sony DSLR 24.6 MP image sensor will have even smaller microlenses. However, these microlenses are fill factor enhancing lenses, which are very small (e.g., with a lateral scale of microns) and can be fabricated by standard lithographic means. It is not possible to use such techniques for fabricating imaging lenses (such as, e.g., the objectives of mobile phone cameras), which are several orders of magnitude larger, because the magnitude of lens sag is significantly higher, e.g., on the order of hundreds of microns. Accordingly, the only technique currently available to form these lenses is by diamond turning. Currently, large arrays of these imaging lenses are either fabricated by full wafer diamond turning, or by the so-called step & repeat technology of duplicating identical lenses across a wafer.
As the numbers of individual lens elements required has increased, it has become difficult to ensure proper quality control using these standard techniques. Specifically, during manufacture of the master lens template, even by a state-of-the-art process such as diamond turning, lenses are formed with different shapes, and therefore, inherently different optical properties even when the lenses were intended to be identical. This shape deviation from the ideal lens profile results in wavefront errors and finally in a reduced image resolution. In addition, it is difficult to maintain pitch control (i.e., the relative placement of lenses in the x-y plane) when performing such manufacturing across an entire wafer of lens elements. The result is that it is very difficult to manufacture a master lens array that is close to ideal, i.e., that has properly shaped lenses (no shape deviation), and that are also properly positioned in relation to the other lens elements (good pitch accuracy.
Moreover, while these standard techniques are designed to produce large arrays of identical lenses, in state-of-the-art computational array cameras irregular lens arrays (meaning lenses within one array having different surface profiles) are required, for example, to correct chromatic aberrations of the different channels sensitive to different narrow spectral wavebands. Current manufacturing techniques provide no alternative but to individually diamond turn each of the unique lens elements, which, again, increases the probability that non-ideal master arrays will be formed, i.e., that include one or more shape or pitch deviations.
Accordingly, a need exists for fabrication processes capable of efficiently and accurately achieving highly precise large regular, but mainly irregular lens arrays, which have imaging lenses with such large sags that the very “original.” or “initial.” master structure, even for the ideal lens, can be done only by diamond turning.